fortran_vec: Fix instances of method chaining that may cause dangling pointer. * libinterp/corefcn/quandcc.cc (Fquadcc): Replace method chaining with a separate line for NDArray object to avoid going out of scope. * libinterp/corefcn/__ilu__.cc (F__iluc__): Obtain RowVector and ColumnVector objects in separate line to remain in scope. * libinterp/corefcn/__ichol__.cc (F__icholt__): Obtain RowVector object in separate line to remain in scope. For more details: https://octave.discourse.group/t/3132

%!function out = bug54490 ()
%!  global k;
%!  k = 1;
%!  out = 3;
%!endfunction

%!test <*54490>
%! global k;
%! k = 2;
%! a = [5, 6];
%! a(k) = bug54490 ();
%! assert (a, [5, 3]);
%! k = 2;
%! a = [5, 6];
%! [a(k)] = bug54490 ();
%! assert (a, [5, 3]);
%! clear -global k;  # cleanup after test